Heterogeneous networks are becoming popular due to the rapid increase of numbers of mobile subscribers and demand for bandwidth, and the inadequacy of traditional macro base stations in meeting subscriber requirements. Homogenous networks consisting of solely traditional macro base stations may have blind spots in coverage that may adversely impact user experience. With the introduction of lower power base stations, including pico cells, femto cells, and relay nodes, newer generation of wireless network topology such as that of a LTE-A network becomes a heterogeneous network (HetNet) that is able to deliver more complete coverage and to support diverse types of wireless devices. In a HetNet as defined in 3GPP Release 10, low power nodes (LPNs), such as Remote Radio Unit/Remote Radio Head (RRU/RRH), pico eNB (Enhanced Node B), home eNB, and relay node, are deployed inside the macro base station or enhanced node B coverage cell.
The LTE technology is adapted for a smooth evolution from earlier 3GPP systems. In a LTE networking environment, technologies such as inter-cell interference coordination (ICIC) in the frequency domain and enhanced ICIC (eICIC) in the time domain have been developed for a new heterogeneous network topology in LTE-Advanced (LTE-A) technology. In a heterogeneous network deployment, combined usage of eICIC and cell range expansion (CRE) can be effective means for improving the system and cell-edge throughput. With eICIC, a macro cell may utilize almost blank subframes (ABS) with zero transmission power mainly in Physical Downlink Control Channel/Physical Downlink Shared Channel (PDCCH/PDSCH) to mitigate the interference to pico user equipments (UEs) with Cell Range Expansion (CRE). Furthermore, a resource status mechanism may enable a pico eNB to provide information in order to help the macro eNB evaluate the need for modification of the ABS pattern. To this end, a macro eNB may determine the ABS pattern adjustment based on downlink (DL) ABS status information.
The concept of an Almost Blank Subframe (ABS) was introduced in eICIC to address control channel interference between a macro eNB and a smaller base station such as a pico eNB in the time domain. Almost blank subframes are transmitted at low power and only contain limited signals. The interfering base station is configured to include ABSs in its transmission so that the ABS may be used by the interfered cell to provide service for a UE that previously experienced strong interference. By coordinating the transmissions of the macro eNB and the pico eNB using ABS, inter-cell interference is minimized or avoided. An aggressor eNB is an interfering eNB and a victim eNB is an interfered eNB.
In a HetNet deployment with small cells, it is likely that the traffic may be fluctuating, since the number of users per small cell node is typically not very large due to small coverage. In a small-cell coverage, it is likely that the user distribution is also very fluctuating and dynamic between the small cell nodes. Allowing for asymmetric uplink (UL)-downlink (DL) allocations has been claimed as one of the benefits of using TDD system. The asymmetric resource allocation in LTE TDD is realized by providing seven different semi-statically configured uplink-downlink configurations. These allocations may provide between 40% and 90% DL subframes. In current LTE deployment, same TDD configuration in each cell may be assumed, because otherwise interference between UL and DL including both base station-to-base station and UE-to-UE interference needs to be considered. However, in local area (LA) network, due to small number of active UEs per cell, the traffic situation may fluctuate frequently, and TDD reconfiguration to adapt to the traffic may provide improved resource efficiency and power saving. Some recent small cell enhancement proposals further point out it is of practical use that coordination between small cells and between small cell and macro cell is necessary to provide sufficient robustness of joint transmissions, efficient resource allocation and etc.
Enabling of the flexible TDD configuration due to the traffic fluctuation may cause more variable DL transmissions at a small cell depending on the traffic situation. This hence may require the relatively frequent increase/decrease of protected resource at a macro eNB. However, for a small cell, located in the overlapped area of two or more different aggressor cells, it may be difficult to coordinate the ABS allocation between these aggressor cells. Especially the assumption/limitation that only the common available ABS from the different aggressor cells should be adopted, may cause a small cell victim UEs (edge UE in the CRE range) to suffer from the severe resource restriction, in respect to the variable DL transmission due to the flexible TDD configuration. It in turn further results in the aggravation of the frequent increase/reduction of protected resource at macro eNB to meet its request.
Some current time domain eICIC solution standardized for HetNet is subject to limitations concerning the possibility to coordinate ABS allocation across different aggressor eNBs. Hence a simple and feasible solution is needed to allow an eNB to adapt the allocated ABS pattern in a way to not only match other aggressor eNB patterns but also minimise resource wastage caused by the currently specified unusable ABSs. It may also facilitate the reduction of unnecessary frequent adjustment of protected resource for meeting the resource request in such case.
In current eICIC, it is possible that multiple aggressor cells allocate ABS patterns or ABS resource to victim cells. However, it is possible that the victim cell is unable to use the ABS subframes allocated by the aggressor cell, due to high interference caused by other aggressors that have not allocated the same ABS pattern. In another word, when there are multiple interfering macro cells to one pico cell, pico cell may receive multiple ABS patterns from these associated macro cells. Since it is possible that there is no/few common subsets among those ABS patterns transmitted from neighboring macro cells to the pico cell, there is consequently no/few usable ABS resource for the pico UEs in the CRE area of this specific pico cell for data transmission.
With the number of densely distributed small cells increasing, many unusable ABS may be created due to poor ABS coordination among aggressor cells. In addition to the usable ABS resource, other resource may also be used to reduce the resource waste in some specific situations. For example, when a pico cell with a large CRE bias is unevenly located in the overlapped area of two aggressor macro cells, the victim UE in the CRE region of pico (located in the major aggressor cell, but far away from the other aggressor cell) may use the ABS allocated by the major aggressor cell rather than the other secondary aggressor cells. Sometimes the assumption may be relaxed that only the commonly available ABS can be the usable ABS. For example the assumption may be less strict when a first priority ABS, and a second priority ABS are differentiated. The ensuing resource status feedback and measurement subset impact may be considered. Via such a modified Usable primary ABS Pattern Info and secondary ABS pattern Info IE, an aggressor eNB may be able to understand which ABS subframes are actually adequately used by the victim cell, independently of the aggressor cell. Hence it may provide the aggressor cell a full view of the allocated protected resource utilization. In this way, the macro eNB may re-arrange its ABS patterns in a way that may better match the patterns of other aggressor nodes and satisfy the small cell requirement for the protected resource and therefore minimise both macro and pico resource wastage.
In contrast, if the ABS is adjusted solely based on the resource status of the primary usable ABS, it may be unfair from the whole system point of view, especially when the pico cell with fluctuating traffic is unevenly located in the overlapped area of two aggressor macro cells. To ensure an eNB to adapt the allocated ABS resources in a way to not only match other aggressor eNB resources but also minimise resource wastage caused by the currently specified unusable ABSs, a scheme is desired that allows the macro cell to consider both primary and secondary ABS statuses in deciding whether or not the protected resource allocation may be adjusted.
Following abbreviations are used in this application.
ABS almost blank subframe
AIL Acceptable Interference Level
BS Base Station
CPICH Common Pilot Channel
CRE Cell Range Expansion
DL Downlink
DPCCH Dedicated Physical Control Channel
E-DCH Enhanced Data Channel
ECNO Received Energy Per Chip/Power density in Band
EUTRAN Enhanced UTRAN
eICIC Enhanced Inter-Cell Interference Coordination
eNB Enhanced Node B.
FDD Frequency Division Duplex
HS-DPCCH High Speed-Dedicated Physical Control Channel
LAS LP-ABS status
LP-ABS Low Power Almost Blank Subframe
LTE Long Term Evolution
OAM Operation, Administration and Maintainence
PDCCH Physical Downlink Control Channel
PDSCH Physical Downlink Shared Channel
PRB Physical Resource Block
RCC Radio Resource Control
RLC Radio Link Control
RNTP Relative Narrowband Tx Power
RRC Radio Resource Control
RSRP Reference Signal Receiving Power
RSRQ Reference Signal Received Quality
RRU/RRH Remote Radio Unit/Remode Radio Head
Rx Receive
TDD Time Division Duplex
Tx Transmit
UE User Equipment
UMTS Universal Mobile Telecommunications System
UTRAN UMTS Radio Access Network
WCDMA Wideband Code Division Multiple Access
ZP-ABS Zero Power Almost Blank Subframe